1. Field of the Invention
This invention relates generally to the power supply systems that include DC-to-DC conversion operations. More particularly, this invention relates to an improved circuit design and configuration of a forward type DC-to-DC converter with resonant reset.
2. Description of the Prior Art
Forward type topology is widely used in DC-to-DC converters due to the simplicity of the structure. In high input voltage operation, dual switch forward topology is used to reduce the voltage stress of main switches. Due to the limitations of magnetic reset mechanisms, dual switch forward topology is not suitable for wide input voltage operations.
FIG. 1A and FIG. 1B respectively show the configuration and key operation waveforms of a dual switch forward DC-to-DC converter of the prior art. Two switches are employed in the primary side. When the two switches turn on, the transformer primary winding is connected to the input voltage and the energy is delivered from source to load. When the two switches turn off, the magnetizing current passes by the two clamping diodes, which are denoted as Da1 and Da2 in FIG. 1A. The input voltage is applied to the primary winding reversely and the magnetizing current is reset to zero. Since the drain-to-source voltage of the switches-is clamped to the input voltage, the switches only endure the voltage stress of the input.
However, since the reset voltage is equal to the input voltage, the reset time is also equal to the turn-on time of the switches in order to keep the voltage-second balance for the transformer. Thus, the maximum switching duty cycle is limited to less than 50% for low input operation conditions. With the increase of the input voltage, the duty cycle becomes small and the performance of the converter deteriorates.
For reducing the conduction loss of the primary side and lowering the voltage stress of the secondary side, it is desirable to increase the duty cycle of the forward converter to greater than 50%. If a resonant reset mechanism is provided in the forward converter, the duty cycle can be designed over than 50% since the reset voltage can be higher than input voltage.
FIG. 2A and FIG. 2B respectively show the configuration and key operation waveforms of a single-ended forward DC-to-DC converter with resonant reset of the prior art. In this converter only one switch S1 is employed in the transformer primary side, and a resonant reset capacitor Cr is connected in parallel with the switch S1. When the switch S1 turns on, the transformer primary winding is connected to the input voltage Vin, and the energy is delivered from source to load by the transformer coupling. When the switch S1 turns off, the magnetizing current charges the resonant capacitor Cr, and the voltage of capacitor Cr increases and resets the transformer core. After a half of resonant period the magnetizing current is reset to zero and the voltage of the primary winding remains zero due to the cross conduction of the secondary rectifier. The voltage of capacitor Cr maintains as input voltage Vin until the switch S1 turns on. When switch S1 turns on, the capacitor Cr is discharged through S1, and the energy stored in capacitor Cr is dissipated in switch S1. Thus, the power loss of switch S1 becomes larger. Especially for high input voltages, the power loss of switch S1 increases significantly because the energy stored in capacitor Cr increases with the square of the input voltage.
The other disadvantage is that the voltage stress of the switch S1 is the sum of the maximum reset voltage and input voltage, which is about double of input voltage. For these reasons, this topology is only suitable for low input voltage and low power applications.
Therefore, a need still exists to provide a new and improved power converter topology that combines the advantages of dual switch forward and resonant reset forward, but overcomes the disadvantages of the prior art.
It is accordingly an object of the invention to provide a novel and improved resonant reset dual switch forward DC-to-DC converter topology, which overcomes the above-mentioned disadvantages of the prior art. In the preferred embodiment of the present invention, a resonant reset dual forward converter comprises: an input for accepting a DC voltage; a transformer having a primary winding and a secondary winding; two main switches connected in series with the primary winding of the transformer for periodically connecting the input to the primary winding; a resonant capacitor for resetting the transformer during the OFF time of the main switch; an auxiliary switch remaining OFF during the ON time of the main switch, and connecting the primary winding to the resonant capacitor during the OFF time of the main switch; and a rectification circuit connecting the secondary winding to the output.
Due to the large reflected output current, the low side main switch and auxiliary switch turn on under a zero-voltage condition; so a larger resonant capacitor can be used to reset the transformer.
In a further embodiment of this invention, an extra inductor is connected in series with the primary winding or the secondary winding to obtain a zero-voltage-switching condition for the high side switch.
In another embodiment of this invention, a center-tapped S1 rectification circuit is employed. The output inductor can be reduced significantly, and the output voltage ripple is minimized.
In another embodiment of this invention, a current doubler rectification circuit is employed. The output inductor is divided into two smaller ones and the secondary winding need not be tapped. The output voltage ripple is also minimized.
In another embodiment of this invention, a synchronous rectifier circuit is employed to reduce the rectification loss. Meanwhile, a control circuit is needed to maintain the freewheeling rectifier on during the OFF time of main switches.
An advantage of the present invention is that it provides the ability to enlarge the switching duty cycle and reduce the conduction loss of the primary switches.
Another advantage of the present invention is that it obtains a soft switching condition for the main switches. Thus, it provides a converter with high power efficiency.
Another advantage of the present invention is that it requires lower rating voltage switches, so that it can be used for high input voltage conversion design.